Optical switches are commonly used in fiber optic communication systems for switching an optical signal from one optical fiber to another. An optical cross-connect is an optical switch that includes multiple input and/or output ports and has the ability to provide, for the purposes of signal transfer, various input port/output port combinations.
Optical cross-connects are often broadly classified according to the optics used to route the light and perform the switching (e.g., waveguide or free-space). Optical cross-connects based on the deflection of free-space light beams have emerged as excellent candidates for use in optical fiber communication systems due to their relatively low insertion loss, low cross talk, and insensitivity to wavelength and polarization. Free-space based optical cross-connects typically include multiple input ports and output ports connected across a switch core, which has the ability to connect, for purposes of signal transfer, any input port to any output port.
The switch core typically uses one or more arrays of beam-steering elements. For example, in many free-space based systems the switch core will include one or more deflector arrays, wherein each deflector array includes an array of tiltable mirrors fabricated using micro-electromechanical systems (MEMS) technology. In general, the input and output ports will be physically located on opposite sides of the switch core for direct or folded optical pathway communication therebetween, or will be located on the same physical side of the switch core facing a mirror (e.g., in side-by-side matrices or interspersed in a single matrix arrangement).
Referring to FIG. 1, a schematic diagram of one prior art optical switch core is shown. The optical switch 100 includes an input fiber array 116, an output fiber array 118, collimators 114, and two deflector arrays 112. A light beam transmitted from an input fiber of input fiber array 116 is switched to a selected output fiber of output fiber array 118 along a folded Z-shaped optical path through the switch 100. The individual deflectors 110 on the deflector arrays 112 provide the deflection along the folded optical pathway, thus allowing for a more compact switch design.
Referring to FIG. 2, a schematic presentation of another prior art optical switch core is shown. The optical switch 200 includes an input fiber array 216, an output fiber array 218, two deflector arrays 212, and an angle-to-offset (ATO) lens 220. The ATO lens 220, which has a focal length f, is disposed in the center of the switch core between the first and second deflector arrays 212. More specifically, the first and second deflector arrays 212 are disposed at a focal plane of the ATO lens 220. The ATO lens 220 provides a re-imaging and deflects the propagation path of light within the switch core. As is discussed in U.S. Pat. Nos. 6,487,334, 6,711,316, and 7,039,267, which are hereby incorporated by reference, the use of an ATO element provides an optical switch having reduced aberrations and/or that is relatively compact.
In the optical switch cores illustrated in FIGS. 1 and 2, each deflector array 112/212 is a 2D array of N2 dual axis MEMS mirrors. For example, in one embodiment, each mirror is a silicon or polysilicon mirror mounted with a gimbal suspension. The number of ports achievable is limited by the diameter of the light beams incident on the MEMS mirrors and the range through which the MEMS mirrors can rotate/tilt. For example, the tilt angle may be limited by the snap-down phenomenon, whereas the diameter of the light beams may be limited by the optical layout and the size of the individually addressable MEMS mirrors. In each optical switch core 100/200, the light beams incident on the individual MEMS mirrors have a round cross-section (e.g. there is a beam waist at each of the individual MEMS mirrors in the array). In addition, the switch cores are designed such that the light beams incident on the MEMS mirrors in the MEMS arrays 112/212 have the same diameter within each switch. For example, referring to FIG. 3, which schematically illustrates the beam size through the switch core 100, it is clear that the configuration causes the illumination spots on each micro-mirror in the array 112 to have a same diameter (wm). In addition, the switching directions of the two deflector arrays 112 are parallel, and the switching directions of the two deflector arrays 212 are parallel.